Groundwater Quality in the Stream-Valley Aquifers of the Ohio and Allegheny Valleys

Groundwater supplies nearly 50 percent of the nation's drinking water. To help protect it, the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Project studies the quality of groundwater in aquifers that are important sources of drinking water. The stream-valley aquifers of the eastern United States are one of them.

The aquifers

The stream-valley aquifers studied lie under about 9,600 square miles along the Ohio and Allegheny River drainages and parts of a few large tributaries, in Pennsylvania, West Virginia, Ohio, Kentucky, Indiana and Illinois. About 2.8 million people live above them, and in 2000 about 200 million gallons a day were withdrawn from them for public supply in these states. Most of the land above them is undeveloped (60 percent); about 27 percent is agricultural and about 13 percent urban. Major cities include Pittsburgh, Cincinnati, Louisville and Evansville.

The study area and the sampled wells. Credit: U.S. Geological Survey.

The aquifers are Holocene sand and gravel laid down as alluvium in the valleys of major rivers. Some of it is reworked glacial material carried downstream by rivers such as the Allegheny and Ohio, which rise in glaciated areas. The aquifers are typically hydraulically connected to the river in their valley. Groundwater is commonly under water-table (unconfined) conditions, but confined where clay or silt forms local confining layers. Recharge comes from precipitation soaking in and from surface water draining from the adjacent streams and rivers, and the lock and dam systems that regulate much of the rivers may affect how surface water moves into the aquifer.

How the study worked

The USGS sampled 55 public-supply wells spread at random across an equal-area grid, a design that makes the data representative of water quality across the whole study area; the accuracy of the estimates depends on the number and spread of wells, not on the size of the area. The wells were about 45 to 135 feet deep, averaging about 86 feet, and were sampled between June and September 2019 for a large number of natural and man-made substances.

The study looks at untreated groundwater. Water reaching customers is usually treated first, so it may differ.

Judging the results

Concentrations are compared with drinking-water benchmarks: federal regulatory standards for health where they exist, and otherwise non-regulatory health and aesthetic benchmarks.

• High: above a human-health benchmark, or above a secondary maximum contaminant level (SMCL), a non-regulatory benchmark for taste, colour, odour, staining or scaling.
• Moderate: for inorganic constituents, above half a benchmark; for organic constituents, above a tenth, because organic compounds are generally less common and occur at lower concentrations relative to benchmarks.

Overview of water quality as a percentage of the study area. Credit: U.S. Geological Survey.

Overview

Inorganic constituents with human-health benchmarks — many of them natural, though human activity can affect them — were at high concentrations in about 38 percent of the study area and moderate in about 20 percent. Man-made organic constituents, found in household, business, industrial and farm products, were detected infrequently and at low concentrations; none with a human-health benchmark was found at moderate or high levels.

Inorganic constituents with health benchmarks

• Trace elements and major and minor ions. Of 34 analysed, 22 have human-health benchmarks. High or moderate concentrations were found in 49 percent of the study area, with five constituents elevated in all. Manganese was the most frequent — high in 33 percent and moderate in 14 percent — and arsenic was moderate in 2 percent.
• Radioactive constituents. Most radioactivity in groundwater comes from the decay of natural uranium and thorium. Of eight analysed, four have health benchmarks. They were high or moderate in about 4 percent of the area: gross-beta radioactivity at high concentrations, and radon at a moderate concentration in one sample.
• Nutrients. Naturally low in groundwater; higher levels generally come from fertiliser on crops and landscaping, septic systems, and human and animal waste. Of five analysed, two — nitrate and nitrite — have health benchmarks. Nitrate was the only one elevated: high in 2 percent and moderate in 4 percent of the area.

Constituents with non-health benchmarks

Eleven constituents with SMCLs were analysed; one or more was high in about 65 percent of the area and moderate in about 29 percent.

• Total dissolved solids, a measure of salinity from dissolved minerals that can also rise with road salting and some farming, were high in about 6 percent and moderate in about 84 percent.
• Iron and manganese, released from minerals when groundwater lacks oxygen, were high relative to the SMCL in about 27 and 66 percent respectively.
• pH was outside the SMCL range of 6.5–8.5 in about 5 percent.

Benchmarks for the constituents found at high concentrations:

MCLs are the Environmental Protection Agency's regulatory maximum contaminant levels; HBSLs are non-regulatory health-based screening levels.

Organic constituents

• Volatile organic compounds, found in many household, commercial, industrial and farm products and quick to evaporate: 85 analysed, 51 with health benchmarks; none at moderate or high concentrations.
• Pesticides, including herbicides, insecticides and fumigants used on crops, gardens, lawns, buildings and roadsides: 225 compounds and breakdown products analysed, 119 with health benchmarks. They were detected at low concentrations across much of the area, but none at moderate or high concentrations.

Sources

Based on "Groundwater Quality in Selected Stream-Valley Aquifers, Eastern United States," U.S. Geological Survey Fact Sheet 2022–3037 (June 2022), National Water Quality Program, published by the U.S. Geological Survey; rewritten in hubnx's own words.

• The background, overview, map, charts and benchmark table come from the fact sheet's PDF: https://pubs.usgs.gov/fs/2022/3037/fs20223037.pdf
• The fact sheet's summary counts of all inorganic and organic constituents analysed do not match the sums of its category counts, so they are left out.